content: add blog cover images and use original Chinese descriptions

- Download 17 blog post cover images from Wayback Machine archive
- Replace auto-generated summaries with full Chinese text from old site
- Show cover images in blog cards with natural aspect ratio (no crop)
- Add download script for reproducible image fetching

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: brian033